The present invention relates to a remote-plasma-CVD process for coating or treating a large-surface substrate in which a coating gas is excited by an excitation gas, which has been excited remotely from the substrate to be coated or treated and by means of a plasma.
Plasmas, which are excited by action of electrical energies of different frequencies--from null (direct voltages) to microwave frequencies--, are often used to treat substrate surfaces. The electromagnetic energy can act then so that the reactant gas plasma is provided in direct contact with the substrate to be treated, so that the reactant gas decomposes into reactive species, which act on the substrate surface, or so that the plasma is produced in an excitation gas remote from the substrate surface and the excitation gas excites the reactant gas. In the latter case a so-called remote plasma (other designations include: down-stream plasma and afterglow plasma) is involved.
Remote-plasma-CVD processes are known and are described many times in the literature. In a remote-plasma-CVD process for coating of substrates an excitation gas ("gas type A") comprising a non-coating forming gas or a mixture of several non-coating forming gases passes through a discharge zone, in which excited and atomic species are formed. The term "non-coating-forming gas" means e.g. noble gases, O.sub.2, H.sub.2 and N.sub.2 O. This excitation gas is mixed with a coating-forming gas("gas type B") comprising a single coating-forming gas or several gases, of which at least one is a coating-forming gas in a discharge-free region (Afterglow) remote from the excitation source. The term "coating-forming gas" means a so-called precursor gas, e.g. TiCl.sub.4 or SiH.sub.4.
The gas actually used to treat or coat the substrate is formed by the excitation gas and the coating-forming gas together. The coating-forming gas, separately from the vise excitation gas, may also pass through an excitation zone; its composition and excitation field strength must be selected in this case so that it is not pre-reacted or is non-coating.
In a region in which the excitation gas and the coating-forming gas are mixed an interaction between the excitation gas and the coating-forming gas occurs, which essentially causes a transfer of excitation energy of molecules or atoms from gas type A to molecules or atoms of gas type B and causes a homogeneous pre-reaction. The pre-reacted components then react heterogeneously with the substrate and form the coating on it.
The advantages of the remote, in contrast to the conventional, plasma CVD process, in which the plasma is in direct contact with the substrate surface to be treated or coated are:
The substrate undergoes no radiation damage in remote-plasma-CVD process, since it is not exposed to the high energy components from the plasma.
For the same reason substrate heat up is at most insignificant.
A conductive coating can be made without difficulties, since, in contrast to other non-remote apparatuses, microwave windows are so far from the coating region that they are not coated with a conductive coating and thus not blocked for further transmission of microwaves.
A conductive substrate can be coated without difficulty, since the fields of a plasma-exciting radiation are not influenced by it.
During excitation by means of microwaves a potential microwave-field distribution from the discharge zone to the substrate does not occur in practice, because of equalizing diffusion processes in the excitation gas on the path between excitation location and substrate.
The corresponding applies to the treatment of substrate surfaces, where by by "treatment an etching or property change on the surface should be understood (e.g. wettability).
Generally the coating or treatment of substrate surfaces should be made as uniform as possible. Usually the plasma is produced by microwave discharge. In this case difficulties occur in the uniform coating or treatment of substrate surfaces, when the dimensions of the substrate surface are on the order of the wavelengths used or more.
German Patent Document DE 39 23 188 C2 discloses a remote-plasma-CVD process for making thin layers on large-surface substrates. DE 39 23 188 C2 teaches uniform distribution of an excitation gas and a coating-forming gas over a path which extends over the width of the reaction chamber and formation of a laminar flow over the substrate by flow engineering techniques. On the same path the excitation gas is converted into the plasma state by microwave radiation passing through the window of an elongated resonator whose width corresponds to that of the rectangular hollow wave guide providing the microwave radiation and widening like a funnel, it is mixed with the coating-forming gas in the reaction chamber.
In this apparatus the following problems occur: The microwaves supplied to the resonator can interfere with each other and excite the excitation gas differently periodically over the outlet width. This periodicity results in differences in the properties of the deposited coating, which depend on the degree of excitation. The reactor is fixed; coating properties may not be influenced by geometric parameters of the reactor. Furthermore the rectangular hollow wave guide can be widened only to a limited extent, so that it is not possible to coat a substrate with a larger surface area to be coated in this reactor.
GB-PS 222 60 49 describes a remote-plasma-CVD apparatus for making a thin coating on a stationary substrate. The exhaust gas from the reaction is drawn out through a gas outlet under the substrate. The substrate may be coated by flowing the excitation gas plasma formed in a microwave hollow wave guide through a complicated shower arrangement in the direction of a coating-forming gas, which similarly flows from a gas shower device comprising a perforated pipe or tube in the direction of the substrate. So that a uniform coating-forming gas distribution over the entire substrate surface is obtained in spite of differing pressure conditions in the pipe or tube, the outlet holes in the perforated pipe or tube must be of differing diameters. The uniformity of the coating adjusted in this manner is attained only for a fixed process parameter relationship, which sets the relationship of the values of the coating region, gas mass flows, gas type, process pressure and gas pressure. Another parameter choice or other substrate size requires significant pre-experimentation to newly find the conditions required to obtain a uniform gas distribution. Correspondingly it is not possible to increase the excitation gas spray or shower indefinitely without limit in order to be free in the size of the coating region since the excitation degree of the gas in its edge region decreases to strongly away from a certain size of the gas shower.
DE 39 26 023 C2 discloses a linear remote plasma source, in which the plasma for the excitation gas is produced in a microwave wave guide. The excitation gas flows through a glass tube and forms an interior conductor in a coaxial microwave wave guide in the plasma state. The excitation gas can issue through a longitudinal slot in the glass tube, exciting a laterally flowing coating-forming gas and thus causing the coating process to occur. Since both the microwave power drop and loss occurs in the excitation gas on the z-axis of the excitation tube, the coating is necessarily nonuniform in this coordinate. An additional disadvantage is the limited length of this apparatus.
FIG. 3 of this patent shows an embodiment with two parallel operable microwave arrangements which are arranged behind each other. This embodiment operates using Photo-CVD excitation. Also this apparatus has the same above-described disadvantages as the individual sources.
DE 38 30 249 C2 describes a process for coating of large-surface substrates by means of a "direct" plasma, in which a plurality of plasma electrodes arranged in a grid-like arrangement over the substrate surface generate a large-surface plasma zone, preferably by microwave excitation, comprising a plurality of overlapping plasma columnar regions. Although very good results were obtained with this process, edge effects in the vicinity of the overlapping plasma columnar regions are not completely avoided.